Iodate method for purifying plutonium



IODATE METHOD FOR PURIFYING PLUTONIUM Application December 30, 1944 Serial No. 570,802

14 Claims. (Cl. 23-14-5) N Drawing.

Our invention relates to the separation and purification of transuranic elements, and particularly to the separation of elements 93 and 94 from radioactive elements of lower atomic numbers. This invention also relates to new and useful compounds of elements 94 utilized in or produced by the separation procedure.

An object of our invention is to provide means for separating certain dangerously radioactive elements from element 94.

A further object of our invention is to provide means for concentrating element 94 from dilute solutions thereof.

Additional objects and advantages of our invention will be evident from the following description.

In this specification and claims, the name of the element designates generically the element both in its free state and combined in a compound. The element in its free state is designated by the term elemental or by its specific state, such as metallic.

The designation element 94 is used throughout this specification to refer to any isotope of the element having an atomic number of 94. The designation 94 means the isotope of element 94 having a mass number of 239. Element 94 is also referred to in this specification and probably will become known in the art as plutonium, symbol Pu. Likewise, element 93 means an element having the atomic number of 93. Element 93 is also referred to as neptunium, symbol Np. Reference herein to any of the elements is to be understood as denoting the element generally whether in its free or combined state unless otherwise indicated by the context.

The apparent discovery of transuranic elements (93 and elements of higher number) was first announced by Fermi in 1934. At that time Fermi stated that the bombardment of uranium with neutrons gave beta activities which he attributed to transuranic elements of atomic number 93 and possibly higher. From 1934 to 1938 other workers, notably Hahn and Curie extended this work. But in 1939 Hahn and Strassman discovered that the elements which they and others had believed to be transuranic elements were in fact radioactive medium atomic weight elements or fragments produced by the fission or splitting of the uranium atom. This discovery of fission resulted in great activity by scientists throughout the world. Hahn and Strassmans results were confirmed, and a great many fission products in addition to those first identified were subsequently discovered and identified. The fission fragments were all of lower atomic number than uranium, generally of atomic numbers near the middle of the periodic group; and so far as we know, prior to about June 1940, no positive evidence was found indicating the existence of any transuranic element.

However, in June 1940, McMillan and Abelson published in The Physical Review, 57, 1185 (1940), their discovery that a 2.3 day activity produced by the bombardment of uranium with neutrons was an isotope of element 93, probably 93 Although McMillan and Abelson surmised that element 94 would be formed by beta decay from element 93 they were unable to produce any positive evidence of its existence, and did not obtain either 93 or 94 in pure form or in macroscopic amounts either as the element or as a compound.

E. Segre, G. T. Seaborg and J. W. Kennedy, using the methods of McMillan and Abelson, obtained 93 admixed with rare earths, proved that 93 decayed to 94 and measured the radioactive and fission properties of 94 In accordance with the present invention, applicants have discovered and developed methods adapted for obtaining element 94 substantially free from fission products such as rare earth elements which are dangerously radioactive and are also undesirably absorbent of neutrons. In such a process a solution of plutonium is formed and certain of the fission products removed to a substantial degree by precipitation upon contact with an insoluble iodate which absorbs or carries fission products from the solution. These methods will be hereinafter described. First we shall describe the reactions which take place by the action of neutrons on uranium to produce the irradiated uranium masses with which we are particularly concerned in our separation processes.

Natural uranium comprises largely isotope U together with about M as much U and a very much smaller amount of U When this or other mixture of these isotopes, either as metallic uranium or as a uranium compound, is subjected to bombardment by neutrons from an external source, or undergoes a self-sustaining neutronic chain reaction, a number of nuclear reactions take place. Isotope U captures a neutron to form U which undergoes beta decay to form transuranic elements:

23 min. (1

U238+ l U239 93239 Isotope U undergoes fission, i. e., a breakdown of its heavy nucleus into lighter fragments which are generally very unstable and highly radioactive. Such fragments usually undergo beta-particle disintegration in successive steps, leading ultimately to stable isotopes of higher nuclear charge than the original fragments. The fission of U is predominantly binary, and may be exemplified by the following type of equation:

Substantially all of the fission fragments have mass numbers within the range 77-158, although small quantities of isotopes of lower and higher mass numbers may result from unbalanced binary fissions, ternary fissions, or other reactions of infrequent occurrence. A large majority of the fission fragments comprise a light group of mass numbers 84-106 and a heavy group of mass numbers 128-150.

The various decay products of the initial fission fragments are referred to herein as fission products. These fission products fall within a range of atomic numbers from about 32 to about 64. The fission products from the light group of fragments have atomic numbers ranging from about 28 to about 48; and the fission products from the heavy group of fragments have atomic numbers ranging from about 49 to about 65.

The various radioactive fission products have half-lives ranging from a fraction of a second to thousands of years. Those having very short half-lives may be eliminated by aging the material for a reasonable period before handling. Those with very long half-lives do not have sufiiciently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission products having half-lives ranging from a few days to a few years have dangerously intense radiations which cannot be eliminated by aging for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, and Ru of the light group and Te, I, Cs, Ba, La, Ce and Pr of the heavy group.

The total amount of 93 and 94 produced from U in natural uranium by neutron bombardment is a function of neutron density and of the time of bombardment. Since 94 is fissionable under the conditions for fission of U the concentration of 94 will build up to a maximum and thereafter will tend to decompose substantially as rapidly as formed. For this reason, a neutronic reaction for 94 production is suitably terminated when only a fraction of the U has been converted to fission products. The reaction mass at this point contains a large amount of U a much smaller amount of U an amount of 93 and/or 94 not in excess of the initial U concentration and fission products; and traces of other products such as UX and UX By aging such a mass for a suitable period of time, the 93 may be largely or substantially completely converted to 94 with simultaneous conversion of the short-lived radioactive fission products to longer-lived or stable isotopes.

The plutonium concentration of the aged reaction mass will usually be less than 0.1% of the weight of the unreacted uranium, and is frequently as low as one part per million parts of uranium. The concentration of fission products will be of the same order of magnitude. The separation of plutonium from such as mass involves extraction from the unreacted uranium, decontamination by separating the radioactive fission products, and con centration of the decontaminated material to obtain a product from which a relatively pure plutonium compound can be directly recovered.

In accordance with our invention, a mass of uranium may be subjected to the action of neutrons, preferably with neutrons of resonance or thermal energies, to produce a mass containing element 93. The mass may be aged to allow element 93 to decay to element 94 at least to a substantial degree preferably until most of the element 93 has decayed to the element 94. This aging may occur to a suitable degree during the neutron bombardment if such bombardment has been continued for a suitable period, for example one or more months. However, additional aging may be desirable in any case in order to reduce the amount of highly radioactive short life fission products. The element 94, accompanied by element 93, if present, is then separated from foreign products such as fission fragments and products, particularly rare earths and other substances which are highly absorbent of neutrons.

In accordance with the present invention we have found that fission products may be removed from a solution containing plutonium in the higher state of oxidation without removal of substantial plutonium by means of thorium iodate or similar insoluble iodate. This removal may be secured in solutions containing relatively high concentrations of excess iodate ion, thus making possible substantially complete precipitation of insoluble iodates which will carry or adsorb only the fission products to a substantial degree.

Although plutonium has an oxidation number of plus 3 (Pu this state is relatively unstable and the normal oxidation state is plus 4 (Pu+ although under certain conditions, i. e. in a dilute acid solution even this state is unstable undergoing dismutation to the plus three or '4 plus six state. The normal plutonium ion is therefore Pu, which is referred to herein as the plutonium ion. Plutonium has at least one higher state of oxidation, namely plus 6 corresponding to PuO referred to herein as the plutonyl ion. Although there is some evidence that other states of oxidation above plus 4 may exist, it is probable that the higher state of oxidation employed in the present invention is primarily the hexavalent state, and for purposes of illustration we shall refer to the plutonyl ion as representing such higher state of oxidation.

In accordance with our invention the higher state of oxidation of plutonium is utilized to maintain the iodate in soluble form while precipitating an insoluble iodate which will carry impurities, especially lower atomic weight elements of undesirable radioactivity. For this purpose an acidic solution of the plutonium and lower radioactive elements is formed, and the plutonium is maintained in the higher state of oxidation while contacting the solution with a substantially insoluble, solid, finely divided metal iodate. This iodate may be added in the preformed solid state, or the solution may be passed through an adsorption column containing this composition. However it is preferable to form an iodate precipitate in situ in order to obtain substantially more complete carrying of radioactive elements. This precipitation may be effected by introducing into the acidic solution of hexavalent plutonium a cation whose iodate is acid-insoluble, and subsequently introducing iodate ion in a concentration at least equivalent to the concentration of such cation.

The cations which may be employed to form the insoluble iodates may be any metal ions whose iodates are known to be insoluble in acidic solutions, and which do not react adversely or form complexes with the other components of the solution to be treated. A large number of cations are available for this purpose but we generally prefer to employ thorium (Th+ and cerium (Ce+ ions. The zirconyl ion is also useful in our process although its ionic constitution is not definitely established. In most cases, however, thorium or ceric iodate, or a combination of the two, Will effect the best separation of radioactive elements from plutonium.

The concentration of cation to be employed may vary over a wide range, but in any case should be substantially in excess of the total concentration of radioactive elements to be separated. Generally speaking, the degree of separation obtained will be improved with increasing amounts of iodate precipitate, so that the upper limit of cation concentration will be determined only by economic considerations due to cost of reagents and volumes of materials to be handled.

In order to maintain the hexavalent plutonium in solution during precipitation of the iodates, the solution should have a relatively high acidity and preferably should have a pH not substantially above 2.0. For this purpose the plutonium and lower radioactive elements may be initially dissolved directly in a strong acid, such as a mineral acid or a highly ionized organic acid which does not form complexes with heavy metal ions. In general we prefer to employ mineral acids such as nitric, hydrochloric, hydroiodic, sulphuric, and iodic acids. If a precipitate is desired which contains only iodate anions, it will be apparent that certain acids will be excluded depending on the choice of cation for the precipitation of the insoluble iodate. Thus, if thorium iodate is to be precipitated free from other anions, orthophosphoric acid cannot be employed to adjust the pH of the solution since thorium phosphate would be precipitated in addition to thorium iodate. Other considerations, such as the formation of complex ions, may exclude other acids in certain instances. However, nitric and hydrochloric acids are highly satisfactory in all cases, and it is unnecessary to attempt to use other acids for the adjustment of the pH of the solutions. In the case of these two acids we prefer to employ solutions of normalties ranging from 2 to 3.

After the plutonium and lower radioactive elements are initially dissolved in a strong mineral acid to produce the solution from which the iodate is to be precipitated, the plutonium ion normally will be the plutonous ion which will require oxidation to the higher state before effecting the precipitation in such solutions. Approximately 1.1 volts are required to oxidize plutonium from the tetravalent to the hexavalent state, the Value of 1.1 volts being the potential in the system of standard oxidation potentials which is referred to the hydrogen-hydrogen ion couple as zero. While, in general, suitable oxidizing agents can be chosen byreferring to a table of oxidation agents, with their oxidizing potentials, such as given by Latimer and Hildebrand in the Handbook of Chemistry and Physics, in certain instances more powerful oxidizing agents may be required than are indicated by potential alone. Thus, in the presence of sulphuric acid somewhat more powerful agents may be required than in the presence of most of the other acids discussed above. As examples of suitable oxidizing agents for use in our process there may be mentioned the alkali metal bismuthates, alkali metal permanganates, alkali metal dichromates, and lead dioxide, either as Pb0 per se, or in the form of Pb O To effect the oxidation, a quantity of oxidation agent at least equivalent to the amount of plutonium is added to the solution, and the resulting mixture is digested at a moderately elevated temperature for a sufiicient period of time to insure complete oxidation of the plutonium. In most cases the digestion is suitably elfected at 60 to 70 C. for to minutes.

After the oxidation of the plutonium is complete the solution is preferably cooled to room temperature, and iodate ion is then introduced to effect the desired precipitation. The iodate ion may be added in any suitable form, e. g., as an alkali metal iodate or as iodic acid. A substantial excess of iodate ion is desirable in order to secure complete precipitation, and we generally prefer to use a concentration of iodate ion 2 to 3 times the equivalent concentration of the added cation. Following the addition of the iodate ion, and sufiicient agitation to secure adequate dispersion, the mixture is allowed to stand for a sufficient time for the insoluble precipitate to form. This period of time may vary somewhat, depending upon the temperature and the concentrations involved, but is preferably as short as possible consistent with complete separation. From 10 to 30 minutes at room temperature will generally be satisfactory, and we prefer to employ a period of approximately 20 minutes. The resulting precipitate may then be separated by any of the usual procedures such as filtering, decanting, centrifuging, or the like.

The precipitate obtained as above described contains a large proportion of the highly radioactive elements which were originally associated with the plutonium, and leaves the latter in a solution which can be more safely handled for further purification and concentration. The procedure may be repeated for a number of cycles by concentrating the supernatant liquid from each precipitation and adding further cation and iodate ion to effect a subsequent precipitation. Alternately, the procedure may be combined with other types of purification or concentration steps. One aspect of our present invention comprises a combination of the separation of soluble plutonyl iodate, as described above, with the separation of insoluble plutonous iodate. This combination is particularly advantageous for the recovery of plutonium from neutron irradiated uranium, and this phase of our invention will be illustrated with particular reference to the separation of plutonium from this source.

Neutron irradiated uranium, containing relatively large amounts of unchanged uraniuin and small amounts of plutonium and fission products may be suitably dissolved in a strong acid to obtain a solution of pH not substantially above 2, from which the insoluble plutonous iodate may be precipitated. The types of acids and acid concentrations which may be employed are the same as those previously described for the separation of the water soluble plutonyl iodate. The preferred solution being an aqueous nitric acid solution 2 to 3 normal with reference to nitric acid. The plutonium in the solution will normally be in the tetravalent state, but a small amount of hydrogen peroxide may be introduced to insure reduction of any plutonium which may be present in a higher state of oxidation, without simultaneously effecting reduction of the uranyl ion.

If the concentration of plutonium in the resulting solution is sufiiciently high, iodate ion may then be added to effect precipitation of plutonous iodate. In most cases, however, it will be advantageous to introduce a larger amount of another insoluble iodate by precipitation or other means to insure complete carrying of the plutonous iodate. For this purpose any of the cations may be used which were previously discussed in connection with the separation of the soluble plutonyl iodate. The concentration of the added cation should be substantially in excess of the concentration of the plutonous ion. lodate ion is then introduced in a concentration preferably at least 2 to 3 times the equivalent total concentration of plutonous ion and added cation, and the resulting precipitate is separated within a short period of time, before a substantial quantity of uranyl iodate can precipitate. The separation of the precipitate is preferably effected in as short a time as possible after the introduction of the iodate ion, consistent with complete carrying of plutonium. A period of about 10 minutes between addition of the iodate and the filtering or centrifuging will be satisfactory in most cases.

The precipitate obtained as described above contains substantially all of the plutonium content of the initial mixture, together with some of the radioactive fission products and a small percentage of the initial uranium. This precipitate may then be dissolved in a strong acid (preferably hydrochloric acid) to form a solution from which an insoluble iodate may be precipitated to remove fission products carried down by the iodate precipitate leaving plutonyl iodate in the supernatant liquid in accordance with the procedure which has previously been described. In this manner a two operation cycle enables the separation of substantially all of the uranium, and high proportion of the highly radioactive elements, from the plutonium. Further purification and concentration of the plutonium may be effected by reducing the plutonyl to the plutonous ion and. repeating the cycle as many times as desired. For this purpose the plutonyl ion may be reduced to the plutonous state by the use of reducing agents having a negative potential which must be more positive than 1.1 volts in the Latirner & Hildebrand table of the standard oxidation and reduction potentials previously referred to. Hydrogen peroxide is one of the preferred agents for this purpose.

After a plurality of cycles in which insoluble plutonous iodate is first separated and soluble plutonyl iodate is then separated, there is finally obtained a solution of plutonyl iodate of relatively high purity, containing radioactive elements of lower atomic numbers only in such minute amounts that the solution may be safely handled for the ultimate recovery of plutonium. Such acid solutions of plutonyl iodate are valuable for the recovery of masses of plutonium or plutonium compounds suitable for use in neutronic reactors for the production of atomic energy. Such solutions may be concentrated to obtain plutonyl iodate in hydrated crystal form, or may be subjected to electrolysis or otherwise treated to obtain plutonium in the elemental or metallic form.

Although our invention has been discussed above with particular reference to plutonium it should be understood that the separation procedures described are equally elfective for the separation of mixtures of neptunium and plutonium. In most cases, however, we prefer to employ as starting material a mixture which has been allowed to age for a suflicient time so that substantially all of the neptunium has decayed to plutonium so that the latter is the predominant component obtained as the final purified iodate.

Our invention will be further illustrated by the following specific example.

Example Plutonium was separated from the uranium and fission products contained in uranyl nitrate hexahydrate which had received 100 milliampere hrs. neutron bombardment. The uranyl nitrate had been stored for approximately four weeks after bombardment, and it contained a substantial amount of 94 Pa but was practically free from 93Np The 94Pu was separated by the following procedure:

Approximately 1053 parts by weight of the bombarded uranyl nitrate hexahydrate described above, and approximately 30 parts by weight of thorium nitrate dodecahydrate, were dissolved in sufficient nitric acid to produce a solution 2N with respect to nitric acid after the addition of 3186 parts by weight of a 0.35 M potassium iodate solution. Sufificient radioactive plutonium, 94Pu Was incorporated as a tracer to give an at count of 10,000 per minute per ml. of the final mixture. The potassium iodate solution was then added, producing a solution having a uranium concentration of approximately 0.050 g. per ml. This solution, containing the resulting thorium iodate precipitate, was allowed to stand for twenty minutes at room temperature.

The thorium iodate precipitate, containing the bulk of the plutonium, was then filtered off and washed with a solution of 1.0 M with respect to nitric acid and 0.1 M with respect to potassium iodate. The washed precipitate was dissolved in 1188 parts by weight of 12 N hydrochloric acid. 2198 parts by weight of 0.5 M sodium dichromate solution was added, and the resulting solution was then diluted with water to a concentration 2.4 N with respect to hydrochloric acid and 0.1 M with respect to sodium dichromate. This solution was then digested for one-half hour at 65 C. to effect oxidation of the Pu to Pu+ The Pu+ solution was then cooled to room temperature, after which 4248 parts by weight of 0.35 M potassium iodate solution was added, and the mixture was allowed to stand for twenty minutes at room temperature. The resulting thorium iodate precipitate, containing the bulk of the fission products, was filtered oil and washed in the same manner as the first thorium iodate precipitate.

The distribution of the plutonium and fission products between the first thorium iodate precipitate, the first supernatant liquid, the second thorium iodate precipitate, and the second supernatant liquid, was determined by measurement of the a, B, and T radiation emitted. For this purpose, blank determinations were first made on the original mixture, prior to the separation of the first thorium iodate precipitate. The a count on this original mixture was taken to be that of the added 94Pu Aliquots were analyzed for total a count, and for total ,8 count corrected for the UX B count, by means of Geiger-Mueller counters and well known techniques. Aliquots of the two thorium iodate precipitates and of the two supernatant liquids were then analyzed for p and 11 activities in the same manner.

The plutonium content of the two thorium iodate precipitates and of the two supernatant liquids was recovered by an additional precipitation in each case, and the or activity of each of the precipitates was then determined. Since 94Pu and 94Pu have identical chemical properties, the distribution of 94Pu as indicated by the a counts, also represented the distribution of the 94Pu The distribution of uranium, plutonium, and fission products obtained by the above separation procedure is shown in the following table:

Cerium iodate may be used in lieu of thorium iodate with substantially identical results.

It is to be understood, of course, that the above example is merely illustrative and does not limit the scope of our invention. Other mixtures of plutonium, or plutonium and neptunium, with radioactive elements of lower atomic numbers may be employed as starting materials; and other acids and other cations may be substituted for those of the above example, within the scope of the foregoing description. Likewise, the process of the example may be modified by incorporating other purification steps in addition to the use of one or more cycles of purification by the separation of soluble plutonyl iodate, or one or more dual cycles of purification by the separation of insoluble plutonous iodate and the separation of soluble plutonyl iodate. In general, it may be said that the use of any equivalents or modifications of procedure which would naturally occur to one skilled in the art is included in the scope of our invention. Only such limitations should be imposed on the scope of this invention as are indicated in the appended claims.

We claim:

1. In a process for the separation of iodate-insoluble radioactive elements from a solution of plutonium and said radioactive elements of lower atomic weights, the steps which comprise maintaining said plutonium in a state of oxidation higher than the tetravalent state and maintaining the pH of said solution not substantially above 2, while contacting said solution with a substantially water insoluble, solid, finely divided metal iodate, and separating said iodate and associated radioactive elements of lower atomic weights from the plutoniumcontaining solution.

2. In a process for the separation of iodate-insoluble radioactive elements from an inorganic acid solution of plutonium and said radioactive elements of atomic numbers within the ranges of about 35-45 and 51-58, the steps which comprise maintaining said plutonium in the hexavalent state and maintaining the pH of said solution not substantially above 2, while contacting said solution with a thorium iodate precipitate, and separating said thorium iodate precipitate together with associated radioactive elements from the plutonium-containing solution.

3. In a process for the separation of iodate-insoluble radioactive elements from an inorganic acid solution of plutonium and said radioactive elements of atomic numbers within the ranges of about 35-45 and 51-60, the steps which comprise maintaining said plutonium in the hexavalent state and maintaining the pH of said solution not substantially above 2, while contacting said solution with a ceric iodate precipitate, and separating said ceric iodate precipitate together with associated radioactive elements from the plutonium-containing solution.

4. In a process for the recovery of plutonium from a mixture of plutonium and iodate-insoluble radioactive elements of lower atomic numbers, the steps which comprise forming an acidic aqueous solution containing said plutonium and radioactive elements and a cation whose iodate is substantially acid-insoluble, maintaining the plutonium in said solution in a state of oxidation higher than the tetravalent state while introducing iodate ion in a concentration at least equivalent to the concentration of said cation, and separating the resulting iodate precipitate and its associated radioactive elements from the supernatant plutonium solution.

5. In a process for the recovery of plutonium from a mixture of plutonium and iodate-insoluble radioactive elements obtained from uranium fission, the steps which comprise forming an aqueous inorganic acid solution of pH not substantially above 2, said acid being a member of the group consisting of hydrochloric acid and nitric acid, containing said plutonium and radioactive elements and a cation whose iodate is substantially insoluble in said acid solution, maintaining the plutonium in said solution in the hexavalent state While introducing iodate ion substantially in excess of the concentration equivalent to the concentration of said cation, and separating the resulting iodate precipitate and its adsorbed and coprecipitated radioactive iodates from the supernatant plutonium solution.

6. In a process for the recovery of plutonium from a mixture of plutonium and iodate-insoluble radioactive elements of atomic numbers within the ranges of about 35-44 and 51-58, the steps which comprise forming an aqueous nitric acid solution at least 1N with respect to nitric acid, containing said plutonium and radioactive elements and a cation whose iodate is substantially insoluble in said acid solution, the concentration of said cation being substantially in excess of the total concentration of said radioactive elements, maintaining the plutonium in said solution in the hexavalent state while introducing iodate ion substantially in excess of the concentration equivalent to the concentration of said cation and separating the resulting iodate precipitate and its adsorbed and co-precipitated radioactive iodates, from the supernatant plutonium solution.

7. In a process for the recovery of plutonium from a mixture of plutonium and iodate-insoluble radioactive elements of atomic numbers within the ranges of about 35-44 and 51-58, the steps which comprise forming an aqueous hydrochloric acid solution at least 1N with respect to hydrochloric acid, containing said plutonium and radioactive elements and a cation whose iodate is substantially insoluble in said acid solution, the concentration of said cation being substantially in excess of the total concentration of said radioactive elements, maintaining the plutonium in said solution in the hexavalent state while introducing iodate ion substantially in excess of the concentration equivalent to the concentration of said cation, and separating the resulting iodate precipitate and its adsorbed and co-precipitated radioactive iodates from the supernatant plutonium solution.

8. In a process for the recovery of plutonium from a mixture of plutonium and iodate-insoluble radioactive elements of atomic numbers within the ranges of about 35-44 and 51-58, the steps which comprise forming an aqueous hydrochloric acid solution 2-3 N with respect to hydrochloric acid, containing in said hydrochloric acid solution said plutonium and radioactive elements and thorium ion in a concentration substantially in excess of the total concentration of said radioactive elements, maintaining said plutonium in the hexavalent state while introducing iodate ion substantially in excess of the concentration equivalent to the concentration of thorium ion, and separating the resulting thorium iodate precipitate and its adsorbed and co-precipitated radioactive iodates from the supernatant plutonyl iodate solution.

9. In a process for the recovery of plutonium from a mixture of plutonium and iodate-insoluble radioactive elements of atomic numbers within the ranges of about 35-44 and 51-58, the steps which comprise forming an aqueous hydrochloric acid solution 2-3 N with respect to hydrochloric acid, containing in said hydrochloric acid solution said plutonium and radioactive elements and ceric ion in a concentration substantially in excess of the total concentration of said radioactive elements, maintaining said plutonium in the hexavalent state while introducing iodate ion substantially in excess of the concentration equivalent to the concentration of ceric ion, and separating the resulting ceric iodate precipitate, and its adsorbed and co-precipitated radioactive iodates, from the supernatant plutonyl iodate solution.

10. In a process for the recovery of plutonium from neutron bombarded uranium containing small amounts of plutonium and radioactive fission products, the steps which comprise dissolving said bombarded uranium in an aqueous inorganic acid solution, said acid being a member of the group consisting of hydrochloric acid and nitric acid, maintaining the plutonium in said solution in the tetravalent state and maintaining the pH of said solution not substantially above 2 while introducing iodate ion, separating the resulting plutonous iodate precipitate prior to the precipitation of a substantial quantity of uranyl iodate, dissolving the separated precipitate in an aqueous inorganic acid solution, introducing a cation whose iodate is substantially insoluble in said acid solution, maintaining the plutonium in said solution in a state of oxidation higher than the tetravalent state and maintaining the pH of said solution not substantially above 2 while introducing iodate ion, and separating the resulting iodate precipitate and its associated radioactive fission products from the supernatant plutonium solution.

11. In a process for the recovery of plutonium from neutron bombarded uranium containing small amounts of plutonium and radioactive fission products, the steps which comprise dissolving said bombarded uranium in aqueous nitric acid to form a solution 2-3 N with respect to nitric acid, introducing thorium ion substantially in excess of the concentration of plutonium in said solution, maintaining said plutonium in the tetravalent state while introducing iodate ion substantially in excess of the concentration equivalent to the concentration of thorium ion in said solution, separating the resulting thorium iodate precipitate and its adsorbed and co-precipitated plutonous iodate from the supernatant uranyl nitrate solution prior to the precipitation of a substantial quantity of uranyl iodate, dissolving said thorium iodate-plutonous iodate precipitate in aqueous hydrochloric acid to form a solution 2-3 N with respect to hydrochloric acid, oxidizing the plutonium in the resulting solution to the hexavalent state, introducing iodate ion substantially in excess of the concentration of thorium ion resulting from the dissolution of said thorium iodate-plutonous iodate precipitate, and separating the resulting thorium iodate precipitate and its associated radioactive fission products from the supernatant plutonyl iodate solution.

12. The process of removing iodate-insoluble radioactive fission products from a solution containing such products and plutonium which comprises contacting the solution with a water insoluble metal iodate while retaining the plutonium in a state of oxidation above the tetravalent state.

13. The process which comprises removing iodateinsoluble radioactive fission products from a solution containing such products and plutonium by contacting the solution with thorium iodate while maintaining the plutonium in a state of oxidation above the tetravalent state.

14. A method which comprises irradiating uranium with neutrons, permitting the bombarded product to age until a substantial quantity of plutonium has been produced, and separating the plutonium from iodate-insoluble fission products produced by the bombardment by contacting a solution containing the plutonium and fission products with a water insoluble metal iodate while H 12 retaining plutonium in a state of oxidation above the Seaborg et al.: Jour. of Am. Chem. Soc., vol. 70, pp. tetravalent state. 1128-34 (1943); report submitted Mar. 21, 1942, and

this date relied on. References Cited in the file of this patent Langers Handbook of Chem, 5 ed 1 4 1 5 232 Friend: Textbook of Inorganic Chemistry, vol. VII, 5 233, 248, 249; pub. in 1944 by Handbook Pub., Inc., part III, p. 299 (1926); pub. by Chas. Griffin & Co., Ltd., Sandusky, Ohio. London. Seaborg: U. S. Atomic Energy Comm.; declass paper Physical Review, vol. 57, 1940, pp. 1185-1186, Radio- MDDC-SOS, Nov. 14, 1946, page 6. active Element 93, by McMillan et al. 

1. IN A PROCESS FOR THE SEPARATION OF IODATE-INSOLUBLE RADIOACTIVE ELEMENTS FROM A SOLUTION OF PLUTONIUM AND SAID RADIOACTIVE ELEMENTS OF LOWER ATOMIC WEIGHTS, THE STEPS WHICH COMPRISES MAINITAINING SAID PLUTOINUM IN A STATE OF OXIDATION HIGHER THAN THE TETRAVALENT STATE AND MAINTAINING THE PH OF SAID SOLUTION NOT SUBSTANTIALLY ABOVE 2, WHILE CONTACTING SAID SOLUTION WITH A SUBSTANTIALLY WATER INSOLUBLE, SOLID FINELY DIVIDED METAL IODATE, AND SEPARATING SAID IODATE AND ASSOCAIATED RADIOACTIVE ELELMENTS OF LOWER ATOMIC WEIGHTS FROM THE PLUTONIUMCONTAINING SOLUTION. 